This invention relates to a sound reproducing apparatus for converting sound represented by one form of varying energy waveform or signal, suitable for transmission, into another energy waveform or signal.
The apparatus of the invention is particularly suitable for use in the sound distribution system of a drive-in theatre and will therefore be described in relation to such a distribution system. However, it is to be understood that this description in relation to a drive-in theatre sound distribution system is by way of example only and that the apparatus of the invention has other areas of applicability.
Drive-in theatre sound distribution systems generally consist of a central unit which converts the sound track of a film into electrical signals representative of the audio information of the sound track. The sound track may have audio information coded or stored magnetically or optically and the audio information may be monoaural (singlechannel) or multichannel. The electrical signals are conducted, along a wired network, to posts located around the theatre where the signals are reconverted into sound by loudspeakers. The loudspeakers provided are generally of poor sound reproducing quality and only capable of single channel reproduction even though the film sound track may have represented multichannel sound information.
The loudspeakers are mounted on posts located at spaced positions around the theatre and a patron may arrange a speaker by either hooking it onto a window of his car or by merely passing it through an open window of the car so that sound may be heard within the car. Vandalization and accidental damage of speakers is not uncommon and is of concern to theatre operators and owners.
In an attempt at improving sound quality and reducing speaker damage a modified sound distribution system has been developed whereby the film sound track is modulated on a carrier wave and this modulated carrier wave is then transmitted in non-radiating fashion on existing loudspeaker wiring to the various posts around the theatre. The wiring at the post is terminated with connectors or suction cups rather than a speaker. The connectors may be attached to a whip aerial of a radio in the patron's car whereas the suction cups may be attached to the windshield for reception by concealed antennas. The carrier wave is of a frequency that may be tuned to any frequency of the commercial radio broadcast band and thus the signal may be demodulated by the patron's radio and replayed through the radio's sound processing facilities.
In such a system the signal is prone to noise interference from electrical machinery, the atmosphere or radio channels near the portion of the commercial band where the system is tuned to operate.
In addition, with such a radio broadcast system sound may no longer be reproduced by a loudspeaker connected direct to an outlet at a post since the audio information has been modulated. Therefore patrons who do not have radios (either fixed car radios or portable radios) with a whip aerial cannot receive the sound portion of the theatre program. It has been usual for theatre operators to provide these patrons, after payment of a security deposit, with a suitable radio receiver.